Sensing of magnetic fields using fiber optic interferometers has been disclosed in the prior art (see, for example: K. P. Koo et al., "Characteristics of Fiber-Optic Magnetic-Field Sensors Employing Metallic Glass," Optics Letters, Vol. 7, pages 334-336 (July 1982); K. P. Koo et al., "A Fiber Optic DC Magnetometer," IEEE, Journal of Lightwave Technology, Vol. LT-1, No. 3, pages 524-525 (September 1983); and K. P. Koo et al., "A Fiber-Optic Magnetic Gradiometer," IEEE, Journal of Lightwave Technology, Vol. LT 1, No. 3, pages 509-513 (September 1983)). In these devices, a two-arm Mach-Zehnder fiber interferometer is used. One of the fiber interferometer arms serves as a magnetic field sensor on which some magnetostrictive material is bonded or deposited. When exposed to a magnetic field, the magnetostrictive material stretches the sensor fiber while the reference fiber remains unaffected. As a result, a magnetically induced differential path length change or phase shift is introduced at the output of the interferometer. However, such fiber optic magnetometers are sensitive only in one given direction of the magnetic field as defined by the orientation of the sensor element.
In U.S. Pat. No. 4,433,291 (Yariv et al.) an optical fiber for magnetostrictive responsive detection of magnetic fields is disclosed. The strength of the protective magnetic field is determined by standard interferometry techniques by comparing the phase or mode properties of the light from a light source exiting the fiber having a magnetostrictive jacket against the phase or mode properties of light from a light source exiting a reference fiber having pre-determined or identical light transmission properties. The sensitivity of magnetic effect upon the magnetostrictively affected fiber is enhanced by subjecting the magnetostrictive material to a quantitative low-level magnetic field.
In Statutory Invention Registration No. H94 (Koo) issued Jul. 1, 1986, an apparatus for increasing sensing linearity and dynamic range of a fiber optic interferometer-type magnetic field sensor is disclosed. A means for generating a magnetic carrier signal having a frequency greater than a magnetic measured field signal is applied to a magnetostrictive sensing element. A reference signal and measured signal are then sensed and an electrical feedback signal is extracted therefrom to achieve an interferometer output signal which is linearized with respect to the D.C. or low frequency magnetic signal.
In U.S. Pat. No. 4,471,219 (Giallorenzi), an amplitude mode magnetic sensor is disclosed. The sensor detects a magnetic field perturbation while nulling out variations in the signal caused by acoustic perturbations. The system uses first and second optical fibers, an adjustable optical coupler for coupling light therebetween, and a magnetic component attached to the optical coupler for observing a mechanical force thereon proportional to a magnetic field perturbation.
In U.S. Pat. No. 4,378,497 (Giallorenzi), an optical fiber magnetic field sensor with thermal and acoustic isolation is disclosed. This isolation is achieved by a housing which is suitable for use with the device disclosed in U.S. Pat. No. 4,433,291 discussed above.
In U.S. Pat. No. 4,376,248 (Giallorenzi et al.), a sensing element for a fiber optic magnetometer is disclosed. The sensing element is a magnetostrictive material associated with an optical fiber and is adapted for use in an optical fiber Mach-Zehnder interferometer. The magnetostrictive material is preferably nickel or metallic glass.
In U.S. Pat. No. 4,109,199 (Pollonov), a three-axis magnetometer provided with a single calibration checking coil lying in a plane disposed at equal angles to each of the three orthogonal axes of sensitivity is disclosed. Energization of the calibration checking coil with a known current while the calibrated magnetometer is in a known condition of calibration provides sensitivity readings for each of the three axes, which readings provide a standard of comparison for checking the calibration and sensitivity of the magnetometer.
In U.S. Pat. No. 4,450,406 (Bobb), a tri-axial optical fiber system for measuring magnetic fields is disclosed. In the system, a set of three fiber-optic coils are positioned along respective orthogonal axes and have pre-determined lengths and diameters. A source of polarized light is used for transmitting the light subject to Faraday rotation. In another embodiment, light (not necessarily polarized) is transmitted along respective orthogonal axes through a set of three bifurcated fiber-optic cables each forming a reference branch and a substantially equilaterally sensor branch on which a magnetostrictive material is connected. Photo detectors are used in both embodiments to produce current signals indicative of characteristic changes in the transmitted light.
In U.S. Pat. No. 4,849,696 (Brun et al.), an apparatus for determining the strength and direction of a magnetic field is disclosed. This apparatus includes a toroidal or similar body of a non-magnetic, insulating material which is provided with at least three recesses for receiving one magnetostrictive sensor. The associated measuring directions of the sensors are orthogonal to one another. A coil is arranged on the body for applying a magnetic auxiliary field which increases the accuracy and reliability of the measured result.
In U.S. Pat. No. 4,644,273 (Bobb), a magnetic field gradiometer is disclosed. The gradiometer includes a geometric arrangement of fiber optic sensors through which light from a single frequency laser is transmitted. Each sensor is coupled to a magnetostrictive element in the manner of a Mach-Zehnder interferometer with the sensors oriented in orthogonal directions to detect a particular orthogonal component of a magnetic field.
A brief disclosure of a three-axis magnetometer is also briefly described in Conference Proceedings of the IEEE Lasers and Electro-Optic Society (See F. Bucholtz et al., "Three-axis, Optically-Powered Fiber Optic Magnetometer," Conference Proceedings of IEEE Lasers and Electro-Optic Society 1988 LEOS Annual Meeting Nov. 2-4, 1988, No. OS3.2, pages 279-280. The schematic disclosure was for a remotely-addressable magnetically-dithered, three-axis fiber optic magnetometer which uses an all-optical link for interferometer interrogation and dither power. Separate dither frequencies on each axis are used to demodulate the magnetic information from the single interferometer.